how much is my old imac worth? I can’t update my software anymore

Depends on the spec’s and how old it is.

Go to > Apple Trade In - Apple and check.

Also see > What to do before you sell, give away, or trade in your Mac - Apple Support, before selling, donating, passing it down to another user or trading it in.

Have you ever upgraded macOS on your iMac? Your signature is showing a 21.5" iMac with macOS 10.10. The first 21.5" iMac is a Late-2009 model capable of running macOS 10.13 High Sierra. While this is still an old OS, some apps especially web browsers are still supported on it. Later model iMacs can run even newer versions of macOS.

You can get the exact model of your iMac by clicking the Apple menu and selecting "About This Mac". You can use this article to see which versions of macOS are compatible with your specific Mac:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

If your iMac is able to run a newer version of macOS, then you can find various macOS installers within this Apple article which also includes optional instructions for creating a bootable macOS USB installer:

How to create a bootable installer for macOS - Apple Support

Make sure to have a good backup before attempting to upgrade macOS just in case something goes wrong.

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

You can get a trade estimate by going to the "Select your device to get a trade-in estimate." section of Apple Trade In - Apple

Zero. Why not donate it to charity? Or have it recycled by Apple, free.